It is well established that different biologic tissues display significantly different interactions with electromagnetic radiation from the visible and infrared into the microwave region of the electromagnetic spectrum. The photoacoustic effect was first described in 1881 by Alexander Graham Bell and others, who studied the acoustic signals that were produced whenever a gas in an enclosed cell is illuminated with a periodically modulated light source. When the light source is modulated at an audio frequency, the periodic heating and cooling of the gas sample produced an acoustic signal in the audible range that could be detected with a microphone. Since that time, the photoacoustic effect has been studied extensively and used mainly for spectroscopic analysis of gases, liquid and solid samples.
It was first suggested that photoacoustics, also known as thermoacoustics, could be used to interrogate living tissue in 1981, but no subsequent imaging techniques were developed. In Bowen U.S. Pat. No. 4,385,634, ultrasonic signals are induced in soft tissue whenever pulsed radiation is absorbed within the tissue; these ultrasonic signals are detected by a transducer placed outside the body. Bowen derives a relationship (Bowen's equation 21) between the pressure signals p(z,t) induced by the photoacoustic interaction and the first time derivative of a heating functions, S(z,t), that represents the local heating produced by radiation absorption. Bowen teaches that the distance between a site of radiation absorption within soft tissue is related to the time delay between the time when the radiation was absorbed and when the acoustic wave was detected.
The above-referenced U.S. patents and applications filed by the present inventor, detail a diagnostic imaging technique in which pulses of electromagnetic radiation are used to excite a relatively large volume of tissue and stimulate acoustic energy. Typically, a large number of such pulses (e.g., 100 to 100,000), spaced at a repetition interval, are generated to stimulate the tissue. The above-referenced U.S. Pat. No. 5,713,356 discloses methods for measuring the relative time delays of the acoustic waves generated by a sequence of such pulses, and for converting these time delays into a diagnostic image.
The use of small animals, and mice in particular, has become increasing prevalent in laboratory research. Mice, and particularly transgenic mice, have been useful in locating and eliminating causes and treatments for disease. Currently 30 million mice are used in medical research annually. In these applications, it is necessary to examine mice efficiently, in vivo, to detect the condition of the mice in order to assess progress of a study.
Optical fluorescence imaging has frequently been used in imaging transgenic mice. Optical fluorescence imaging allows researches to detect proteins, antibodies and genetic markers in vivo that have been labeled with fluorescing dyes. However, the deleterious effects of optical scattering compromises fluorescence imaging in intact animals, and limits spatial resolution increasingly with the depth of the site of markers within soft tissue.